Inspired by the characters Vera Verb and Norma Noun, I came up with a character called Anthony Antonym, which I used with my Year 3 class. They loved it.

Anthony Antonym is a horrible, obnoxious little boy who does exactly the OPPOSITE of what anyone tells him. So if his teacher tells him to be quiet, he'll be noisy.

He is also a liar, who says the opposite of what he really thinks. I drew him on a sheet of A3 paper, and the idea really seems to have stuck with my children.